I have 2 old (circa 1800?) black and white photos sizes 9x11 inches in glass. It's a picture of a little girl in a flower field and another of a woman outside the home, sitting on a chair with her little girl standing in front, and she is feeding a cat. It is framed, and I set it against our glass window overlooking the lake so that it receives good lighting. It has been there for over 20 years. If it is not against a light, you can not see the picture properly. It is like a glass negative, only it comes out positive against the light. You can see the picture both ways. What kind of photo do I have? Is this an ambrotype photo?

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UnapiedraFeb 24 '12 at 0:02

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@Unapiedra: From how its worded, I don't think the photos are in a digital format...they are physical photos in a frame.
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jrista♦Feb 24 '12 at 2:14

3 Answers
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When you say "against the light", do you mean that the light is shining on or shining through the picture?

If it's shining on, then your pictures are ambrotypes. Like Daguerreotypes, ambrotypes use the silver that's precipitated out during development as a reflective surface. The image is actually a negative; it only appears to be a positive because the silver is more reflective than the black backing that's applied later. If the light falling on the image is such that it's reflecting from the flat black backing (usually a varnish containing lamp black), then the picture will appear to be a negative.

If it's shining through, then they're glass positives; sort of overgrown magic lantern slides that are meant to be seen as the front of a lamp (usually with an oiled paper, parchment or ivory/ivorine (white celluloid) panel behind it).

It sounds like what you have my be a "glass positive", a form of heliography. The heliography process was developed by Joseph-Nicéphore Niépce in 1822 (which would fit the time frame you said the two photographs you have were created), and was a means of reproducing multiple photos via a lithographic process from a single master. Standard heliography would etch the photo into silver-surfaced copper plates.

A glass positive is a form of heliography that used glass plates "sensitized" with silver chloride, rather than a silver-surfaced copper plate. Glass positives are indeed dependent upon lighting and viewing angle. The first glass positives are presumed to have been created by Niépce, however the process was refined by Sir John Herschel in 1839 and again by the Langenheim brothers in 1849 (who apparently made magic lanterns with them.)